1. Field of the Invention
This invention relates to the production of alloy particles for consolidation to produce permanent magnets.
2. Description of the Prior Art
Sintered permanent magnets are produced by practices that include the consolidation and high-temperature sintering of particles constituting the desired alloy composition of the magnet. In the production of sintered permanent magnets including light rare earth elements, and particularly Nd, the particles are produced by a reduction-diffusion practice that includes the use of calcium as the reductant.
In the conventional reduction-diffusion processes for producing particles of permanent magnet alloy, including Nd, for use in consolidation to produce sintered magnets, initially particles of the desired alloy composition and calcium particles are consolidated, as by cold isostatic pressing, to form a bar. The bar is heated for a time at temperature to achieve the desired calcium reduction of the particles of rare earth oxides incorporated in the bar. During this heat treatment, in addition to the calcium reduction of ( The rare earth oxides, the rare earth element, namely Nd, is diffused into the grain boundaries to form a Nd-rich grain boundary phase. It is this phase that is significant n achieving good magnetic properties, particularly coercivity, in magnets subsequently produced from this material Upon the connection of heat treatment, the bar is comminuted, as by a crushing operation, to produce particles therefrom. The particles are usually of -30 mesh (U.S. Standard). The particles resulting from this operation have significant amounts of calcium oxide resulting from the calcium reduction reaction. Conventionally, the calcium oxide is removed by a combination of initial water washing followed by leaching with acetic acid. Mineral acids such as hydrochloric acid and nitric acid are also conventionally used for this purpose.
Applicant has determined that during this acid leaching step significant amounts of the rare earth element, namely Nd, are dissolved to leave the Nd-rich grain boundary phase deficient in Nd. Applicant has further determined that this results in a degradation of the magnetic properties of magnets subsequently made from this material. Specifically in this regard, applicant has conducted experimental work demonstrating that with a conventional reduction-diffusion process for the conventional magnet alloy of NdFeB that acetic acid leaching has the following effect on the composition of the particles:
______________________________________ Nd Dy Fe B Ca O.sub.2 ______________________________________ Nominal Composition 32.0 2.6 64.1 1.3 0 0 Weight % Analyzed Composition 26.5 2.5 65.9 1.3 .16 0.75 Weight % ______________________________________
As may be seen from this analysis of the prior-art practice, a significant amount of Nd (17%) is lost during acetic acid leaching to result in a material that is close to the composition of the Nd.sub.2 Fe.sub.14 B matrix phase (26.8% Nd, 72% Fe, 1.0% B).